Voltage regulators, also known as power converters, provide a regulated output voltage from a voltage source that may be poorly-specified or fluctuating, or that may be at an inappropriate amplitude for the load. Such regulators may employ a switching circuit that includes one or more switching elements coupled in series or in parallel with the load. The switching elements may be, for example, power metal-oxide semiconductor field-effect transistor (MOSFET) switches.
Control circuitry regulates the output voltage and the current supplied to the load by cycling the switch circuit between ON and OFF states. The duty cycle of the switch circuit controls the flow of power to the load, and can be varied by a variety of methods. For example, the duty cycle can be varied by (1) fixing the pulse stream frequency and varying the ON or OFF time of each pulse, (2) fixing the ON or OFF time of each pulse and varying the pulse stream frequency, or (3) a combination thereof.
To vary the ON or OFF time of each pulse or the pulse stream frequency, the control circuitry may monitor the regulator's output voltage and generate a feedback signal Ve that is proportional to the difference between the regulator's output voltage and the regulated voltage. Ve may be used to provide either “voltage-mode” or “current-mode” regulation. In voltage-mode regulation, Ve and a periodic sawtooth waveform Vs may be provided as inputs to a comparator, the output of which controls the duty cycle of the switch circuit. In current-mode regulation, a voltage Vi may be generated that is proportional to the current in the output inductor, and Vi and Ve may be provided as inputs to a comparator, the output of which controls the duty cycle of the switch circuit.
Synchronous switching regulators include at least two switching elements that typically are driven by non-overlapping drive signals to supply current at a regulated voltage to a load. Synchronous switching regulators that use power MOSFET switches frequently are used in portable battery-powered electronic products and thermally-sensitive products. These regulators convert the typically fluctuating input voltage to a regulated output voltage. Such regulators provide high operating efficiency and thus long battery life with little heat generation.
One problem with switching regulators is that the efficiency of the regulator decreases as the load current is reduced to low levels due to switching and quiescent losses. This is particularly problematic in battery-powered applications, which may spend much of their operating time in a low power draw mode (i.e., when load current demand is low).
To maintain high efficiency at light load, thereby increasing battery life, switching power converters may transition from a NORMAL operating mode, e.g., a fixed frequency pulse-width modulation (PWM) mode of operation, to a low power consumption mode in which switching and quiescent losses are significantly reduced. One example of a low power consumption mode is a BURST mode operation, which sometimes also is referred to as a type of pulse frequency modulation with intermittent bursts of pulses. In BURST mode, the voltage regulator operates sufficiently long to bring the output voltage into regulation, after which the converter transitions into SLEEP mode. In SLEEP mode, all switching stops and all the active switching elements are maintained OFF for a period of time that varies as a function of load current, the quiescent current of the converter is reduced, and the load is supported by energy stored in an output capacitor. During SLEEP, additional components of the power converter can be but need not be turned OFF. When the output voltage decreases below a threshold level, e.g., 1% below the regulated value, the converter “awakens” from SLEEP mode and resumes switching until the output voltage again is within regulation limits, at which point the converter resumes SLEEP mode. Alternatively, some switching power converters may transition directly from NORMAL operation to SLEEP mode, maintaining all active switching elements OFF and reducing quiescent current to reduce power consumption. In this manner, high efficiency can be maintained over a wide range of load current since the time spent in SLEEP mode increases as the load current demand is reduced.
When the power converter is subjected to heavier loads, however, NORMAL operating mode, in which the converter switch continuously alternates between an ON state and an OFF state to maintain the output voltage at the regulated level, may be more beneficial than BURST mode operation. Accordingly, the converter may be configured for transition between NORMAL operating mode and BURST mode as the load varies.
Some applications may be able to provide a mode control signal, giving the converter “advance warning” of an impending increase or decrease in load. There are other applications in which a user cannot provide this control, and the converter must be able to switch between modes automatically, based on load, while maintaining regulation.
Preferably, a burst mode control circuit would have means of sensing average load current and switch between modes automatically based on a user-programmed current threshold. It would also respond quickly to sudden load changes, and easily be controlled by the host for manual operation, while requiring a minimum number of components.
There are a number of existing methods for implementing automatic mode control (see, e.g., U.S. Pat. No. 5,481,178 to Wilcox et al.). One method used by current mode converters is to sense load current by monitoring output voltage from a feedback error amplifier. When this voltage drops below a predetermined level, indicating a certain peak inductor current, the converter goes into SLEEP mode. The converter comes out of the SLEEP mode when the output voltage has dropped below a pre-defined threshold. Thereafter, the converter wakes up and resumes switching. The major problem with this method is that the error voltage is an indication of peak inductor current, not average load current. This may cause the mode transition point to vary as much as 10:1, as a function of the VIN/VOUT ratio, inductance, and switching frequency. It also is restricted to converters using current mode control.
Another method is described in the datasheet for Unitrode's UC1874 buck regulator. The UC1874 employs fixed frequency average current mode control and improves light load efficiency with a programmable standby mode during which the MOSFET drivers and the oscillator are disabled. The UC1874 senses average output current by monitoring voltage from a feedback error amplifier. When that voltage, which is indicative of the average output current, drops below a programmable voltage threshold, the converter transitions into the standby mode. One problem with the UC1874 is that the output signal from the error amplifier is accurate only during steady state conditions. During transient conditions, the output voltage from the error amplifier may not be an accurate indication of the average output current due to potential lags in responsiveness that result from employment of average current-mode control.
Another mode control method and circuit is based on the sensing of discontinuous inductor current as an indication of light load. This circuit is not easily programmable, and is subject to large mode threshold variation with VIN, VOUT, inductance, and switching frequency. Furthermore, such a circuit also may require a larger, more expensive inductor having a greater inductance to achieve the desired load transition threshold.
In view of the foregoing, it would be desirable to provide methods and circuits for controlling a voltage regulator having a smooth and repeatable transition between operating modes.
It also would be desirable to provide methods and circuits for automatic transition of a voltage regulator into and out of a low power consumption mode at a mode transition point or threshold that can be set independently of input voltage, output voltage, inductance, and switching frequency.
It further would be desirable to provide methods and circuits for automatic transition of a voltage regulator into and out of a low power consumption mode in which the mode transition point or threshold is easily programmable.
It even further would be desirable to provide methods and circuits for automatic transition of a voltage regulator into and out of a low power consumption mode that may be used in conjunction with voltage mode or current mode control to vary the duty cycle of a switch.
In addition, it would be desirable to provide methods and circuits for transition between operating modes by manual override.